In a number of areas of the world such as the Gulf Coast of the United States, there are geologic formations or zones through which a wellbore penetrates when drilling a well, such as an oil and gas well, which formations or zones contain solid particles that are not strongly held in place. These particles, e.g., fine grained sand, tend to flow into the wellbore as fluids such as oil and/or gas flow from the interior of the formations into the wellbore.
The pumping of a large number of small solid particles through the producing equipment of a well can in some situations cause increased wear and tear on that equipment. Therefore, it is desirable to prevent the production of substantial amounts of solid particles into the wellbore.
One procedure devised to prevent solids production from a well is generically called gravel packing. This procedure involves placing a liner, screen, or other perforated cylindrical device in the area of the wellbore where solid particles are naturally produced from the formation into the wellbore. In the annulus between the outside of the liner and the wellbore wall (face of the formation which is producing solid particles into the wellbore) a particulate material such as sand, not necessarily gravel as the generic term used in the industry implies, is emplaced to act as a filter to keep the finer grained solids produced from the formation from passing through the perforations in the liner and into the wellbore itself.
After forming the pack there is left in the wellbore in the vicinity of the producing formation a liner backed by a pack of solid particles which are sufficiently large so as to bridge or otherwise not pass through the apertures in the liner. The pack particles are sufficiently close packed to filter out finer solid particles being produced from the formation itself without impeding the flow of oil and gas through the pack and liner and into the wellbore for production to the earth's surface. Thus, gravel packs are, in effect, and in situ filtering device so that solid particles entrained in the oil and gas are filtered from it before the oil and gas reaches the interior of the wellbore for production to the earth's surface.
Oftentimes a wellbore has a plurality, i.e. two or more, of packs emplaced therein. This can be necessary because the producing formation is sufficiently thick that a satisfactory pack over the full thickness of the formation requires the emplacement of a series of packs or because more than one formation is producing into the wellbore, or because the producing formation is perforated or otherwise has apertures such as fractures therein which are desirably packed first (referred to in the industry as a pre-pack). In accordance with this invention the term "pack" is intended to cover all packs emplaced in the wellbore itself and all pre-packs which extend into apertures in a formation.
When a plurality of packs are employed in a single wellbore sometimes one pack will leak solids into the wellbore while another pack will not. When it is discovered that, after all packing procedures are completed, the well is still producing solids, it is impossible to know at the earth's surface which pack is not working as desired. Accordingly, it is highly desirable to be able to pin-point the pack or packs which are continuing to leak solid particles into the wellbore so that a workover job can be directed to these particular packs and the others ignored thereby substantially reducing workover costs and rig time necessary to get the well into the desired condition of not producing any substantial amount of solids to the earth's surface.